This invention concerns fuel injector assemblies for gas turbine engines.
There is a continuing need, driven by environmental concerns and governmental regulations, for improving the efficiency of and decreasing the emissions from gas turbine engines of the type utilised to power jet aircraft, marine vessels or generate electricity. Particularly there is a continuing drive to reduce nitrous oxide (NOx) emissions.
Advanced gas turbine combustors must meet these requirements for lower NOx emissions under conditions in which the control of NOx generation is very challenging. For example, the goal for the Ultra Efficient Engine Technology (UEET) gas turbine combustor research being done by NASA is a 70 percent reduction in NOx emissions and a 15 percent improvement in fuel efficiency compared to ICAO 1996 standards technology. Realisation of the fuel efficiency objectives will require an overall cycle pressure ratio as high as 60 to 1 and a peak cycle temperature of 1600° C. or greater. The severe combustor pressure and temperature conditions required for improved fuel efficiency make the NOx emissions goal much more difficult to achieve.
Conventional fuel injectors that seek to address this issue have concentrically arranged pilot and main injectors with the main injector surrounding the pilot injector. However, conventional injector arrangements have several operational disadvantages, including for example, flame stability and re-light characteristics, the potential for excessive combustor dynamics or pressure fluctuations caused by combustor instability. Combustion instability occurs when the heat release couples with combustor acoustics such that random pressure perturbations in the combustor are amplified into larger pressure oscillations. These large pressure oscillations, having amplitudes of about 1-5% of the combustor pressure, can have catastrophic con sequences and thus must be reduced or eliminated.
The invention seeks to provide an improved injector that addresses these and other problems.
According to a first aspect of the present invention there is provided a fuel injector head for a gas turbine engine the head comprising: a pilot injector having a central axis, a main injector located radially outwardly of the pilot injector, a concentric splitter separating the pilot injector from the main injector and bounding a volume through which in use a fuel injected by the pilot injector flows and in which combustion of the fuel is initiated; characterised in that the splitter is internally cooled and has a radially inner surface which defines a first portion which tapers radially inwardly to a throat and a second portion which tapers radially outwardly from the throat, wherein the angle of the radially outwards taper is such that a flow of air in use over the radially inner surface remains attached over the length of the surface.
Preferably the pilot injector comprises an annular pilot fuel housing concentric with the central axis, the inner surface of the fuel housing providing a prefilmer surface for the supply of fuel thereto in the form of a film extending to a prefilmer lip wherein the inner surface defines a bore for the supply of air over the prefilmer.
The pilot injector comprises may comprises an annular outer bore concentric with the central axis for the supply of air over the prefilmer lip.
Preferably the radially inner surface of the splitter provides the radially outer wall of the annular outer bore.
Preferably the concentric splitter has a radially outer wall and a cavity between the radially outer wall and the radially inner surface.